remove legacy Spanner admin interfaces when deprecation period expires

[devbww]

As of 2021-10-01 the hand-wrought

• google::cloud::spanner::InstanceAdminConnection, and
• google::cloud::spanner::DatabaseAdminConnection

connection classes, along with the overloaded functions

• google::cloud::spanner::MakeInstanceAdminConnection(), and
• google::cloud::spanner::MakeDatabaseAdminConnection()

that build them, and the

• google::cloud::spanner::InstanceAdminClient
• google::cloud::spanner::DatabaseAdminClient

client classes, were all deprecated in favor of their similarly named counterparts in the google::cloud::spanner_admin namespace from the https://github.com/googleapis/google-cloud-cpp/tree/main/google/cloud/spanner/admin source tree.

The new interfaces were generated directly from the Spanner admin service definitions in https://github.com/googleapis/googleapis/tree/master/google/spanner/admin by the https://github.com/googleapis/google-cloud-cpp/tree/main/generator. As such, they are at a slightly lower level, dealing only with protobuf types, but the basic elements remain the same.

The old interfaces are now frozen and will be removed from the code base on or shortly after 2022-10-01, so customers will need to migrate any affected code before then.

Deprecation attributes have been added to those old APIs to aid in their identification, while the sample code in https://github.com/googleapis/google-cloud-cpp/blob/main/google/cloud/spanner/samples/samples.cc has been updated to use the new APIs. See the diffs between the old and new samples in #7311 for a more concise picture of how to perform migrations.

[devbww]

And don't forget to re-partition the Spanner test instances, using the "restore-database-partition" label, when the deprecated tests are removed. The "legacy-{core,extra}" instances can be distributed back into the "generated-{core,extra}" instances.

[devjgm]

Waiting until 2022-10-01